It is known that in vacuum technology, and in particular when checking the sealing of chambers with a tracer gas such as helium or hydrogen and a mass spectrometer, it is necessary to use a cock suitable for removing said tracer gas from the chamber which contains a gaseous mixture and in which the pressure may lie anywhere over a wide range, namely, 10.sup.-4 millibars to a few bars.
Conventional cocks enable gas removal from mixtures at a pressure of less than 1 millibar. Above this pressure, there is a danger of damaging the spectrometer. To bleed gas when the pressure of the mixture is higher than 1 millibar, a needle-type valve is disposed in parallel with the main cock and whose aperture is set as a function of the pressure prevailing upstream.
However, such a device is unstable and is liable to close too easily and besides, its structure is relatively complicated.
Further, the gas-analysing mass spectrometer operates at a maximum pressure and therefore in conditions which are incompatible with high reliability.
Preferred embodiments of the invention provide a gas bleed cock which is capable of removing gas from a mixture in a chamber over a wide pressure range, in particular from 10.sup.-4 to a few bars. Preferably such a cock is highly-reliable and is of simple structure.